Display element, writing method and writing apparatus

ABSTRACT

In a display element in which three or more display layers for displaying mutually different color lights are stacked within one pixel and which controls display states of the plural display layers by applying a voltage from the outside of the plural display layers, eight colors—white, black, blue, green, red, cyan, magenta, and yellow—can be displayed within one pixel. Display layers having cholesteric liquid crystals selectively reflecting blue, green, and red lights are stacked between a pair of substrates, and a light absorption layer is formed on the back of the substrate of a non-display side. Threshold voltages of orientation change of the display layers are mutually changed, and a threshold voltage Vpf 90 (A) of change from a planer state to a focal conic state of the display layer having the highest threshold voltage is made higher than a threshold voltage Vfh 90 (C) of change from a focal conic state to a homeotropic state of the display layer having the lowest threshold voltage.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a display element capable ofdisplaying multiple colors, a method of writing images to it, and anapparatus for writing images.

[0003] 2. Description of the Prior Art

[0004] A reflective liquid crystal is suitable as a display element ofsmall-size information equipment, portable information terminals and thelike because it does not require a dedicated light source such asbacklight, has low power consumption, and can be of a thin andlightweight construction.

[0005] There is known a reflective liquid crystal element capable ofdisplaying multiple colors that, between a pair of substrates eachhaving an electrode formed on an inner surface thereof, three liquidcrystal cells forming display layers having cholesteric liquid crystalsselectively reflecting blue, green, and red lights are stacked, and alight absorption layer is formed on the back of a liquid crystal cellopposite to a display side (a side through which outside light comes).

[0006] In the cholesteric liquid crystal display element of cellstacking type, by independently having the cholesteric liquid crystalsof cells switch between a selective reflection state due to a planerstate and a transmission state due to a focal conic state, eightcolors—white, black, blue, green, red, cyan, magenta, and yellow—can bedisplayed within one pixel, and a display with low loss of light andhigh contrast can be obtained because no color filter is used.

[0007] However, the cholesteric liquid crystal display element of cellstacking type has the disadvantages that parallax becomes high becausethere are a substrate and an electrode between display layers ofdifferent colors and the interval between the display layers becomeslarge, and the display element and a display apparatus are expensive tofabricate because driving electrodes and driving circuits for threecolors are required.

[0008] A cholesteric liquid crystal display element capable ofdisplaying multiple colors is proposed in Japanese Published UnexaminedPatent Application No. Hei 10-177191 (hereinafter referred to as a firstconventional example) and Japanese Published Unexamined PatentApplication No. Hei 11-149088 (U.S. patent application Ser. No.09/192,402, hereinafter referred to as a second conventional example).According to the proposed cholesteric liquid crystal display element,three display layers having cholesteric liquid crystals selectivelyreflecting blue, green, and red lights are stacked between a pair ofsubstrates each having an electrode formed on an inner surface thereof,and an image is written and displayed by applying a writing signal fromthe outside of the three display layers.

[0009]FIG. 28 shows a first conventional example. In a display element31 of this example, between a substrate 32 having a writing electrode 34formed on an inner surface thereof and a substrate 33 having a writingelectrode 35 formed on an inner surface thereof, three display layers38A, 38B, and 38C of PDLC (Polymer Dispersed Liquid Crystal) in whichcholesteric liquid crystals 41A, 41B, and 41C selectively reflectingmutually different color lights are respectively droplet-dispersed inpolymeric matrix 42 are stacked, and a light absorption layer 39 isformed on the back of a substrate 33 of a non-display side. Thresholdvoltages of orientation changes of the cholesteric liquid crystals 41A,41B, and 41C are set as described later. The writing electrodes 34 and35 are connected to a writing apparatus (driving circuit) 50.

[0010]FIG. 29 shows a second conventional example. In the displayelement 31 of this example, between the substrates 32 and 33, threedisplay layers 38A, 38B, and 38C having the cholesteric liquid crystals41A, 41B, and 41C selectively reflecting mutually different color lightsare stacked in a way that inserts spacers 37A, 37B, and 37C into thedisplay layers 38A, 38B, and 38C, respectively, and puts a separatingsubstrate 36A between the display layers 38A and 38B and a separatingsubstrate 36B between the display layers 38B and 38C, and the lightabsorption layer 39 is formed on the back of the substrate 33 of thenon-display side. Threshold voltages of orientation changes of thecholesteric liquid crystals 41A, 41B, and 41C are set as describedlater. The writing apparatus 50, which is formed separately from thedisplay element 31, includes the electrodes 54 and 55 sandwiching thedisplay element 31, and a driving circuit 51 for applying a writingsignal between the electrodes 54 and 55.

[0011] A cholesteric liquid crystal having positive dielectricanisotropy has three states: a planer state in which helical axes arevertical to cell surfaces and which causes a selective reflectionphenomenon for incident light, as shown in FIG. 26A; a focal conic statein which helical axes are almost parallel to cell surfaces and whichcauses incident light to transmit while scattering a little forward, asshown in FIG. 26B; and a homeotropic state in which a helical structurecollapses and liquid crystal directors face a field direction and whichcauses incident light to transmit almost perfectly, as shown in FIG.26C.

[0012] The planer state and the focal conic state of the three statescan exist bistably when no electric field is applied. Therefore, theorientation states of cholesteric liquid crystals are not uniquelydetermined for electric fields; when an initial state is the planerstate, as an applied voltage increases, the cholesteric liquid crystalschange in the order of the planer, focal conic, and homeotropic states;and when an initial state is the focal state, as an applied voltageincreases, the cholesteric liquid crystals change in the order of thefocal conic and homeotropic states. On the other hand, if an electricfield is suddenly set to zero, the planer and focal conic states remainunchanged, and the homeotropic state changes to the planer state.

[0013] Therefore, immediately after a pulse signal is applied, thecholesteric liquid crystal layers exhibit an electo-optical response asshown in FIG. 27; when an applied pulse voltage is Vfh90 or more, itenters a selective reflection state representing a change from thehomeotropic state to the planer state; and when an applied pulse voltageis between Vpf10 and Vfh10, it enters a transmission state due to thefocal conic state; and when an applied pulse voltage is Vfh90 or less,it maintains the state in which it was before the pulse signal isapplied, that is, enters the selective reflection state due to theplaner state or the transmission state due to the focal conic state.

[0014] In the figure, the vertical axis represents normalizedreflectivity and normalizes reflectivity by a maximum reflectivity of100 and a minimum reflectivity of 0. Since change of reflectivityentails a transition area, a normalized reflectivity of 90 or more isdefined as a selective reflection state; a normalized reflectivity of 10or less, as a transmission state; threshold voltages of change betweenthe planer state and the focal conic state, as Vpf90 before a transitionarea and Vpf10 after it; and threshold voltages of change between thefocal conic state and the homeotropic state, as Vfh10 before atransition area and Vfh90 after it.

[0015] In the conventional display element 31 shown in FIGS. 28 and 29,these threshold voltages are mutually changed among the display layers38A, 38B, and 38C. Specifically, assuming that threshold voltages of thedisplay layer 38A are Vpf90(A), Vpf10(A), Vfh10(A), and Vfh90(A);threshold voltages of the display layer 38B are Vpf90(B), Vpf10(B),Vfh10(B), and Vfh90(B); and threshold voltages of the display layer 38Care Vpf90(C), Vpf10(C), Vfh10(C), and Vfh90(C), an expression (6) shownbelow is set.

Vpf90(C)<Vpf10(C)<Vpf90(B)<Vpf10(B)<Vpf90(A)<Vpf10(A)<Vfh10(C)<Vfh90(C)<Vfh10(B)<Vfh90(B)<Vfh10(A)<Vfh90(A)  (1)

[0016] The order in which the display layers are stacked is not limitedto the examples of FIGS. 28 and 29. That is, regardless of the order inwhich the display layers are stacked, when the three display layers aredefined as 38A, 38B, and 38C in descending order of threshold voltagesVpf90, Vpf10, Vfh10, and Vfh90, arrangements are made so that thefollowing expression is satisfied

Vpf10(A)<Vfh10(C)  (1a)

[0017] and there are no other threshold voltages between both.

[0018] When there are a refresh period Tr, a select period Ts, and afollowing non-voltage display period Td as shown in FIG. 31, by thewriting apparatus 50, a writing signal representing a voltage selectedfrom the above-described seven voltage levels Va to Vg demarcated by thethreshold voltages as shown in FIG. 30, based on input image data, isapplied between the writing electrodes 34 and 35 or between theelectrodes 54 and 55, holding the relation that a voltage Vr in therefresh period Tr is greater than a voltage Vs in the select period Ts.

[0019]FIG. 32 shows, in this case, the orientation states of the displaylayers 38A, 38B, and 38C by combinations of refresh voltage Vr andselect voltage Vs, wherein “p” designates a selective reflection statedue to a planer state; “f”, a transmission state due to a focal conicstate; and “?”, an undecided state depending on a state before a writesignal is applied. The orientation states indicate the display layers38C, 38B, and 38A from the left in that order.

[0020] As is apparent from the above, according to the conventionaldisplay element 31, the following seven types of orientation states areobtained.

[0021] (1) All of the display layers 38A, 38B, and 38C are the planerstate.

[0022] (2) All of the display layers 38A, 38B, and 38C are the focalconic state.

[0023] (3) The display layer 38A is the planer state, and the displaylayers 38B and 38C are the focal conic state.

[0024] (4) The display layer 38B is the planer state, and the displaylayers 38A and 38C are the focal conic state.

[0025] (5) The display layer 38C is the planer state, and the displaylayers 38A and 38B are the focal conic state.

[0026] (6) The display layers 38A and 38B are the planer state, and thedisplay layer 38C is the focal conic state.

[0027] (7) The display layers 38B and 38C are the planer state, and thedisplay layer 38A is the focal conic state.

[0028] Therefore, for example, on the assumption that the display layers38A, 38B, and 38C selectively reflect blue light, green light, and redlight, respectively, as shown in FIG. 32, the display element can assumethe following seven display states, so that the five colors of white,black, blue, green and red, and the two colors of cyan and yellow, orseven colors in total can be displayed within one pixel.

[0029] (1) White is displayed by a writing signal satisfying relationsof Vr=Vg and Vs=Va.

[0030] (2) Black is displayed by, e.g., a writing signal satisfyingrelations of Vr=Vd and Vs=Va.

[0031] (3) Blue is displayed by a writing signal satisfying relations ofVr=Vg and Vs=Vc.

[0032] (4) Green is displayed by a writing signal satisfying relationsof Vr=Vf and Vs=Vb.

[0033] (5) Red is displayed by, e.g., a writing signal satisfyingrelations of Vr=Ve and Vs=Va.

[0034] (6) Cyan is displayed by a writing signal satisfying relations ofVr=Vg and Vs=Vb.

[0035] (7) Yellow is displayed by, e.g., a writing signal satisfyingrelations of Vr=Vf and Vs=Va.

[0036] In the above example, assuming that the display layer 38B havingintermediate threshold voltages selectively reflects green light, cyanand yellow are displayed as two colors of cyan, yellow, and magenta.However, if it is assumed that the display layer 38B having intermediatethreshold voltages selectively reflects blue light, cyan and magenta canbe displayed as two colors of cyan, yellow, and magenta, and if it isassumed that the display layer 38B having intermediate thresholdvoltages selectively reflects red light, yellow and magenta can bedisplayed as two colors of cyan, yellow, and magenta.

[0037] In the conventional display element 31, except for the thinseparating substrates 36A and 36B of the example of FIG. 29, nosubstrate and electrode are provided between the display layers 38A,38B, and 38C so that the intervals between the display layers 38A, 38B,and 38C become zero or very small, with the result of low parallax andreduced costs of fabricating the display element and display apparatusbecause the writing electrodes and the driving circuit are made commonamong the display layers 38A, 38B, and 38C.

[0038] However, the above-described conventional display element 31 hasthe disadvantages that combinations of orientation states of cholestericliquid crystals 41A, 41B, and 41C of the display layers 38A, 38B, and38C, determined by refresh voltage Vr and select voltage Vs, are no morethan seven types, and the five colors of white, black, blue, green andred, and two colors of cyan, yellow, and magenta, which are determinedby the relationship between the magnitude of threshold voltages of thecholesteric liquid crystals 41A, 41B, and 41C and selectively reflectedcolors, that is, no more than seven colors in total can be displayed,indicating a narrow color reproduction area (color reproduction range).

SUMMARY OF THE INVENTION

[0039] Accordingly, according to the present invention, in a displayelement in which three or more display layers for displaying mutuallydifferent color lights are stacked within one pixel and which controlsdisplay states of the display layers by applying a voltage from theoutside of the display layers, eight colors—white, black, blue, green,red, cyan, magenta, and yellow—can be displayed within one pixel, and acolor reproduction area can be enlarged.

[0040] An aspect of the present invention relates to a display elementhaving three or more display layers each including cholesteric liquidcrystal. The display layers selectively reflect lights of different peakwavelengths, respectively. The layers are stacked within one pixel andhave a threshold voltage of orientation change of the cholesteric liquidcrystals differing from each other for voltage applied from the outsideof the plural display layers. Among the three or more display layers, athreshold voltage of change from a planer state to a focal conic stateof the display layer having the highest threshold voltage is higher thana threshold voltage of change from a focal conic state to a homeotropicstate of the display layer having the lowest threshold voltage.

[0041] Another aspect of the present invention relates to a method ofwriting an image to the display element of the present invention byapplying a writing signal which includes at least a refresh period, aselect period, and a following non-voltage display period. A voltage Vrin the refresh period is greater than a voltage Vs in the select period.

[0042] Another aspect of the present invention relates to a displayelement having three or more display layers including cholesteric liquidcrystals selectively absorbing lights of different peak wavelengths,respectively, by adding dichroic dyes to the cholesteric liquid crystalsor by the dichroism of the cholesteric liquid crystals themselves. Thedisplay layers are stacked within one pixel, and have a thresholdvoltage of orientation change of the cholesteric liquid crystalsdiffering from each other for voltage applied from the outside of thedisplay layers. Among the three or more display layers, a thresholdvoltage of change from a planer state to a focal conic state of thedisplay layer having the highest threshold voltage is higher than athreshold voltage of change from a focal conic state to a homeotropicstate of the display layer having the lowest threshold voltage. In thedisplay element of the present invention configured as described above,for voltages applied from the outside of the plural stacked displaylayers, a threshold voltage Vpf90(A) of change from a planer state to afocal conic state of the display layer A having the highest thresholdvoltage of the three or more display layers A, B, C, . . . is higherthan a threshold voltage Vfh90(C) of change from a focal conic state toa homeotropic state of the display layer C having the lowest thresholdvoltage.

[0043] For this reason, by applying a voltage between the two thresholdvoltages Vpf90(A) and Vfh90(C) to the whole of the plural stackeddisplay layers, orientation states not found in conventional displayelements with threshold voltages set as shown in FIG. 30 are obtained sothat the display layer A having the highest threshold voltage of thedisplay layers A, B, C, . . . and the display layer C having thesmallest threshold voltage go to a planer state and the display layer Bhaving intermediate threshold voltages goes to a focal conic state, sothat eight colors—white, black, blue, green, red, cyan, magenta, andyellow—can be displayed within one pixel, and a color reproduction areacan be enlarged.

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Preferred embodiments of the present invention will be describedin detail based on the followings, wherein:

[0045]FIG. 1 is a view showing a first example of a first embodiment;

[0046]FIG. 2 is a view showing a second example of the first embodiment;

[0047]FIG. 3 is a view showing a third example of the first embodiment;

[0048]FIG. 4 is a view showing a fourth example of the first embodiment;

[0049]FIG. 5 is a view showing a fifth example of the first embodiment;

[0050]FIG. 6 is a view showing a first example of the second embodiment;

[0051]FIG. 7 is a view showing a second example of the secondembodiment;

[0052]FIG. 8 is a view showing a third example of the second embodiment;

[0053]FIG. 9 is a view showing an equivalent circuit of a displayelement of the present invention;

[0054]FIG. 10 is a view showing a first example of electro-opticalresponses of the display element of the present invention;

[0055]FIG. 11 is a view showing a writing signal to the display elementexhibiting the electro-optical responses of FIG. 10;

[0056]FIG. 12 is a view showing the orientation states of the displayelement exhibiting the electro-optical responses of FIG. 10;

[0057]FIG. 13 is a view showing display states of the display element ofthe first embodiment;

[0058]FIG. 14 is a view showing display states of the display element ofthe second embodiment;

[0059]FIG. 15 is a view showing a second example of electro-opticalresponses of the display element of the present invention;

[0060]FIG. 16 is a view showing a writing signal to the display elementexhibiting the electro-optical responses of FIG. 15;

[0061]FIG. 17 is a view showing the orientation states of the displayelement exhibiting the electro-optical responses of FIG. 15;

[0062]FIG. 18 is a view showing a third example of electro-opticalresponses of the display element of the present invention;

[0063]FIG. 19 is a view showing a writing signal to the display elementexhibiting the electro-optical responses of FIG. 18;

[0064]FIG. 20 is a view showing the orientation states of the displayelement exhibiting the electro-optical responses of FIG. 18;

[0065]FIG. 21 is a view showing the orientation states of the displayelement exhibiting the electro-optical responses of FIG. 18;

[0066]FIG. 22 is a view showing reflection reflectivity to a selectvoltage of the display element of the experiment example 1;

[0067]FIG. 23 is a view showing an example of a writing signal when thedisplay element of the experiment example 1 is displayed in eightcolors;

[0068]FIG. 24 is a view showing a reflection spectrum of the displayelement of the experiment example 2;

[0069]FIG. 25 is a view showing a color reproduction area of the displayelement of the experiment example 2;

[0070]FIG. 26 is a view showing orientation change of a cholestericliquid crystal having positive dielectric anisotropy;

[0071]FIG. 27 is a view showing electo-optical responses of acholesteric liquid crystal having positive dielectric anisotropy;

[0072]FIG. 28 is a view showing a first conventional display element anda writing apparatus;

[0073]FIG. 29 is a view showing a second conventional display elementand a writing apparatus;

[0074]FIG. 30 is a view showing electo-optical responses of aconventional display element;

[0075]FIG. 31 is a view showing a writing signal to a conventionaldisplay element; and

[0076]FIG. 32 is a view showing orientation states and display states ofa conventional display element.

DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

[0077] As a first embodiment, a concrete example of making a display byselective reflection of cholesteric liquid crystals will be described.

First Embodiment

[0078]FIG. 1 shows a first example of the first embodiment.

[0079] A display element 1 of this example has three display layers 8A,8B, and 8C including cholesteric liquid crystals (including chiralnematic liquid crystals or chiral smectic liquid crystals) 11A, 11B, and11C selectively reflecting mutually different color lights between asubstrate 2 having a writing electrode 4 on an inner surface thereof anda substrate 3 having a writing electrode 5 on an inner surface thereof,wherein the display layers 8A, 8B, and 8C are stacked on top of eachother with spacers 7A, 7B, and 7C being inserted in the display layers8A, 8B, and 8C, respectively, and with a separating substrate 6Aintervening between the display layers 8A and 8B and a separatingsubstrate 6B intervening between the display layers 8B and 8C, andwherein a light absorption layer 9 is formed on the back of thesubstrate 3 of a non-display side. The writing electrodes 4 and 5 areconnected to a writing apparatus 20.

[0080] The substrates 2 and 3 are made of glass, silicon, or polymericfilms such as polyester (polyethylene terephthalate), polyether sulfoneand polycarbonate, and at least the substrate 2 of a display side ismade of materials having light permeability. As required, a knownfunctional film such as a wear-resistant layer or a barrier layer toprevent gas from invading to the display element 1 may be formed on thesurfaces of the substrates 2 and 3.

[0081] The writing electrodes 4 and 5 are made of an indium tin oxide(ITO) film or the like, and at least the writing electrode 4 of thedisplay side is made of a material having light permeability. Asrequired, a known functional film such as a liquid crystal orientationfilm and an insulating film may be formed on the surfaces of the writingelectrodes 4 and 5.

[0082] As the configuration and the driving system of the writingelectrodes 4 and 5, any of the following is practicable: (1) a segmentdriving system in which one of the writing electrodes 4 and 5 is anelectrode common to pixels and the other is an electrode specific to thepixels; (2) a simple matrix driving system in which the writingelectrodes 4 and 5 are respectively formed in stripe shape in adirection perpendicular to each other and an intersection of both isdefined as one pixel; and (3) an active matrix driving system in whichone of the writing electrodes 4 and 5 is an electrode common to thepixels and the other consists of a scanning electrode and a signalelectrode in stripe shape which are perpendicular to each other,provided with an active element such as TFT and MIM. The same is alsotrue for examples in FIG. 2 and following drawings.

[0083] For the separating substrates 6A and 6B, the same polymeric filmsas for the substrates 2 and 3 can be used and they are made of materialshaving light permeability. It is desirable that they are severalmicrometers to several tens of micrometers in thickness and haveflexibility. To increase the ratio of divided voltages to the displaylayer 8A, 8B, and 8C, as large a dielectric constant as possible isdesirable. As required, a known functional film such as a liquid crystalorientation film may be formed on their surfaces.

[0084] For the spacers 7A, 7B, and 7C, ball-type or cylinder-typespacers made of glass or plastic can be used, and they respectivelycontrol the thickness of the display layers 8A, 8B, and 8C to severalmicrometers to several tens of micrometers. Particularly, when materialshaving flexibility are used for the substrates 2 and 3, in order toprevent the thicknesses of the display layers 8A, 8B, and 8C fromchanging highly due to the deformation of the substrates 2 and 3, it isdesirable that the substrates are bonded to each other using the spacers7A, 7B, and 7C coated with an adhesive component in the vicinitythereof, or movement of the spacers 7A, 7B, and 7C is prevented.

[0085] Instead of the spacers 7A, 7B, and 7C, saliences or the likecapable of controlling the thicknesses of the display layers 8A, 8B, and8C may be formed on the surfaces of the substrates 2 and 3, and theseparating substrates 6A and 6B.

[0086] The light absorption layer 9 is not limited to specific one if itcan absorb incident light transmitting through the display layers 8A,8B, and 8C. Instead of forming the light absorption layer 9 on the backof the substrate 3 of the non-display side, the substrate 3 or thewriting electrode 5 may be provided with light absorbency to eliminatethe light absorption layer 9.

[0087] The cholesteric liquid crystals 11A, 11B, and 11C each havepositive dielectric anisotropy and selectively reflect lights ofmutually different peak wavelengths. The display layers 8A, 8B, and 8Ccan be constituted by cholesteric liquid crystals and polymericcomplexes, or mixed materials of cholesteric liquid crystals andpigments.

[0088] As the cholesteric liquid crystals 11A, 11B, and 11C, thefollowing materials can be used: steroid cholesterol derivatives; chiralsubstances of Schiff base family, azo family, ester family, and biphenylfamily having asymmetry carbons; or materials with these chiralsubstances added to nematic crystal liquids such as Schiff base family,azo family, azoxy family, ethane family, biphenyl family, terphenylfamily, cyclohexyl carboxylic acid ester family, phenylcyclohexanefamily, benzoate ester family, pyrimidine family, dioxane family, tolanfamily, cyclohexyl cyclohexane ester family, and alkenyl family, ormixtures of these.

[0089] If the cholesteric liquid crystals have positive dielectricanisotropy, nematic liquid crystals having negative dielectricanisotropy can also be used.

[0090] Reflection peak wavelengths of the display layers 8A, 8B, and 8Care controlled by, e.g., the helical pitch of the cholesteric liquidcrystals 11A, 11B, and 11C. The helical pitch of the cholesteric liquidcrystals 11A, 11B, and 11C can be adjusted by the chemical structure ofthe respective chiral substances and the ratio of additives to thenematic liquid crystals of the chiral substances.

[0091] The relationship of threshold voltages of orientation change ofthe cholesteric liquid crystals 11A, 11B, and 11C will be describedlater.

[0092] The writing apparatus 20, in this example, includes a voltageapplication part 21 that applies a writing signal between the writingelectrodes 4 and 5 of the display element 1, and a control part 22 thatcontrols the writing signal, based on input image data. The mode of thewriting signal will be described later.

Second Example

[0093]FIG. 2 shows a second example of the first embodiment.

[0094] In this example, the display layers 8A, 8B, and 8C each have aPDLC structure in which the cholesteric liquid crystals 11A, 11B, and11C are droplet-dispersed in a polymeric matrix 12. With the PDLCstructure, the cholesteric liquid crystals 11A, 11B, and 11Cconstituting the display layers 8A, 8B, and 8C do not mix with eachother and the separating substrates 6A and 6B can be eliminated. Thisexample is the same as the example of FIG. 1 in other points.

[0095] The PDLC structure can be formed by known methods such as theemulsion method, the PIPS (Polymerization Induced Phase Separation)method, the TIPS (Thermally Induced Phase Separation) method, and theSIPS (Solvent Induced Phase Separation) method, and the display layers8A, 8B, and 8C are sequentially formed by the printing method or thelike.

Third Embodiment

[0096]FIG. 3 shows a third example of the first embodiment.

[0097] In this example, the cholesteric liquid crystals 11A, 11B, and11C constituting the display layers 8A, 8B, and 8C are transformed inadvance into micro capsules 13 each confined in a polymeric film and aremixed with a solvent and binder 14 as required, and then the displaylayers 8A, 8B, and 8C are sequentially formed by the printing method orthe like. This example is the same as the example of FIG. 1 in otherpoints.

[0098] Although the micro capsules 13 can be formed by known methodssuch as the phase separation (coacervation) method, the interfacialpolymerization method, the solvent removal method, and the in-situpolymerization method, it is desirable that their diameters are made asuniform as possible.

Fourth Example

[0099]FIG. 4 shows a fourth example of the first embodiment.

[0100] In this example, the display element 1 does not internally havethe electrodes 4 and 5, while the writing apparatus 20 is separate fromthe display element 1 and includes the writing electrodes 24 and 25holding the display element 1 between them, the voltage application part21, and the control part 22. Thereby, a writing signal can be applied tothe display layers 8A, 8B, and 8C from the outside of the displayelement 1, so that the display element 1 can be made paper-like.

[0101] To increase the ratio of divided voltages to the display layer8A, 8B, and 8C, it is desirable that the substrates 2 and 3 and thelight absorption layer 9 have as large a dielectric constant aspossible, like the separating substrates 6A and 6B.

[0102] The display layers 8A, 8B, and 8C may have the same PDLCstructure as those in the example of FIG. 2, or may have the same microcapsule structure as those in the example of FIG. 3.

Fifth Example

[0103]FIG. 5 shows a fifth example of the first embodiment.

[0104] In this example, the display element 1 has the display layers 8A,8B, and 8C, the light absorption layer 9, and a photoconductive layer 15stacked on top of each other between the substrates 2 and 3 having theelectrode 4 or 5 formed on an inner surface thereof, while the writingapparatus 20 includes the voltage application part 21, the control part22, and a light irradiation part 27 that irradiates the photoconductivelayer 15 with a writing light 28 through the substrate 3 and the writingelectrode 5 of the display element 1, wherein the writing apparatus 20controls electric fields applied to the display layers 8A, 8B, and 8C incombination of the writing light 28 and a writing signal applied acrossthe writing electrodes 4 and 5 from the voltage application part 21.

[0105] In this example, the substrate 3 and the writing electrode 5 ofthe non-display side are also made of materials having lightpermeability. To increase the ratio of divided voltages to the displaylayer 8A, 8B, and 8C, it is desirable that the light absorption layer 9has as large a dielectric constant as possible.

[0106] Preferably, the photoconductive layer 15 changes in impedanceaccording to the amount of irradiated light; the following can be usedfor the photoconductive layer 15: a film produced by subjecting a chargegeneration substance to the evaporation method, sputtering method, ionplating method, CVD method, or the like; a coat of a charge generationsubstance dispersed across a resin binder, produced by the bar coatmethod, spin coat method, roll coat method, dip method, casting method,or the like; or a charge transport layer stacked on these chargegeneration layers.

[0107] Preferably, the light irradiation part 27 can irradiate thenon-display side of the display element 1 with any amount of the writinglight 28; the following can be used for the light irradiation part 27: aself-generating light element such as a laser beam scanner, LED array,plasma display, EL display, or the like; and a combination of alight-adjusting element such as a liquid crystal shutter, and a lightsource such as a fluorescent tube, xenon lamp, halogen lamp, or mercurylamp.

[0108] The display layers 8A, 8B, and 8C may have the same PDLCstructure as those in the example of FIG. 2, or may have the same microcapsule structure as those in the example of FIG. 3.

Second Embodiment

[0109] As a second embodiment, there is shown a concrete example ofmaking a display by selective absorption of dichroic dyes added tocholesteric liquid crystals.

First Example

[0110]FIG. 6 shows a first example of a second embodiment.

[0111] In this example, the display layers 8A, 8B, and 8C arecholesteric liquid crystals 11A, 11B, and 11C to which dichroic dyes17A, 17B, and 17C selectively absorbing mutually different color lightsare added, and on the back of the substrate 3 of the non-display side, alight scattering layer 18 is formed instead of a light absorption layer.

[0112] Like the first embodiment, the cholesteric liquid crystals 11A,11B, and 11C each have positive dielectric anisotropy. However, since adisplay is made by selective absorption of the dichroic dyes 17A, 17B,and 17C, it is desirable that selective reflection wavelength areas ofthe cholesteric liquid crystals 11A, 11B, and 11C are different fromselective absorption wavelength areas of the dichroic dyes 17A, 17B, and17C; for example, infrared radiation is desirable.

[0113] The relationship of threshold voltages of orientation change ofthe cholesteric liquid crystals 11A, 11B, and 11C will be describedlater.

[0114] As the dichroic dyes 17A, 17B, and 17C, positive dichroic dyesthat a light absorption index in a molecular long axis direction islarger than that in a molecular short axis direction are used.Specifically, as the dichroic dyes 17A, 17B, and 17C, azo-family andanthraquinone-family pigments and other pigments can be used.

[0115] The light scattering layer 18 is not limited to specific one ifit scatteredly reflects incident light transmitting through the displaylayers 8A, 8B, and 8C. Instead of forming the light scattering layer 18on the back of the substrate 3 of the non-display side, the substrate 3or the writing electrode 5 may be provided with light scatteringcapability to eliminate the light scattering layer 18.

[0116] This example is the same as the example of FIG. 1 of the firstembodiment in other points. The display layers 8A, 8B, and 8C may havethe same PDLC structure as those in the example of FIG. 2, or may havethe same micro capsule structure as those in the example of FIG. 3. Inthis case, the dichroic dyes are added to droplet-dispersed cholestericliquid crystals or contained in the micro capsules after being added tothe cholesteric liquid crystals.

Second Example

[0117]FIG. 7 shows a second example of the second embodiment.

[0118] This example shows an external writing method as shown in theexample of FIG. 4 of the first embodiment, combined with the example ofFIG. 6. That is, in this example, the display layers 8A, 8B, and 8Cinclude the cholesteric liquid crystals 11A, 11B, and 11C to whichdichroic dyes 17A, 17B, and 17C selectively selecting mutually differentcolor lights are added, the light scattering layer 18 is formed on theback of the substrate 3 of the non-display side, and writing electrodes24 and 25 are provided in the writing apparatus 20, which is separatefrom the display element 1, to apply a writing signal to the displaylayers 8A, 8B, and 8C from the outside of the display element 1.

[0119] To increase the ratio of divided voltages to the display layer8A, 8B, and 8C, it is desirable that the substrates 2 and 3 and thelight scattering layer 18 have as large a dielectric constant aspossible, like the separating substrates 6A and 6B.

[0120] The display layers 8A, 8B, and 8C may have the PDLC structure ormicro capsule structure.

Third Example

[0121]FIG. 8 shows a third example of the second embodiment.

[0122] This example shows a light combined writing method as shown inthe example of FIG. 5 of the first embodiment, combined with the exampleof FIG. 6. That is, in this example, the display element 1 has thedisplay layers 8A, 8B, and 8C, the light scattering layer 18, and thephotoconductive layer 15 stacked on top of each other between thesubstrates 2 and 3 having the electrode 4 or 5 formed on an innersurface thereof, wherein the display layers 8A, 8B, and 8C includecholesteric liquid crystals 11A, 11B, and 11C to which dichroic dyes17A, 17B, and 17C are added, while the writing apparatus 20 includes thelight irradiation part 27 that irradiates the photoconductive layer 15with a writing light 28.

[0123] In this example, the substrate 3 and the writing electrode 5 ofthe non-display side are made of materials having light permeability. Toincrease the ratio of divided voltages to the display layer 8A, 8B, and8C, it is desirable that the light scattering layer 18 have as large adielectric constant as possible.

[0124] The display layers 8A, 8B, and 8C may have the PDLC structure ormicro capsule structure.

Electo-optical Response and Writing Method

[0125] As described in FIG. 26, a cholesteric liquid crystal havingpositive dielectric anisotropy has three states: a planer state shown inFIG. 26A; a focal conic state shown in FIG. 26B; and homeotropic stateshown in FIG. 26C. Immediately after a pulse signal is applied, acholesteric liquid crystal layer exhibits an electo-optical response asshown in FIG. 27; when an applied pulse voltage is Vfh90 or more, itenters a selective reflection state representing change from thehomeotropic state to the planer state; and when an applied pulse voltageis between Vpf10 and Vfh10, it enters a transmission state due to thefocal conic state; and when an applied pulse voltage is Vfh90 or less,it maintains the state in which it was before the pulse signal isapplied, that is, enters the selective reflection state due to theplaner state or the transmission state due to the focal conic state.

[0126] As described above, however, a normalized reflectivity of 90 ormore is defined as a selective reflection state; a normalizedreflectivity of 10 or less, as a transmission state; threshold voltagesof change between the planer state and the focal conic state, as Vpf90before a transition area and Vpf10 after it; and threshold voltages ofchange between the focal conic state and the homeotropic state, as Vfh10before a transition area and Vfh90 after it.

[0127]FIG. 9 shows an equivalent circuit of the display elements of theexamples of FIG. 1 or FIG. 6. Co and Ro designate an equivalentelectrostatic capacity and an equivalent resistance value of componentsother than the display layers 8A, 8B, and 8C between the writingelectrodes 4 and 5, that is, serial sums of electrostatic capacities andresistance values of the separating substrates 6A and 6B of the writingelectrodes 4 and 5, and Vxo designates a voltage drop developing incomponents other than the display layers 8A, 8B, and 8C when a voltage Vis applied across the writing electrodes 4 and 5 of the display element1 from the writing apparatus 20.

[0128] Ca, Cb, and Cc, and Ra, Rb, and Rc respectively represent theelectrostatic capacities and resistance values of the display layers 8A,8B, and 8C; and Vxa, Vxb, and Vxc respectively designate voltagesactually applied to the display layers 8A, 8B, and 8C. Usually, theresistance values Ra, Rb, and Rc of the display layers 8A, 8B, and 8Care sufficiently large and the electrostatic capacities Ca, Cb, and Ccchange depending on the orientation of liquid crystals because theliquid crystals have dielectric anisotropy.

[0129] When a voltage V is applied across the writing electrodes 4 and5, voltages Vxa, Vxb, and Vxc actually applied to the display layers 8A,8B, and 8C are as shown below.

Vxa=(C/Ca)(V−Vxo)  (2)

Vxb=(C/Cb)(V−Vxo)  (3)

Vxc=(C/Cc)(V−Vxo)  (4)

[0130] where

C=CaCbCc/(CaCb+CaCc+CbCc)  (5)

[0131] In this way, when a voltage V is applied to the display elementof the example of FIG. 1 or FIG. 6, each of the display layers 8A, 8B,and 8C is applied a voltage resulting from the divided electrostaticcapacities as described above, so that the orientation of thecholesteric liquid crystals 11A, 11B, and 11C of the display layers 8A,8B, and 8C changes depending on the voltage.

[0132] This is also the same for the display elements of FIGS. 2 to 5,7, and 8, except that: in the display element of the example of FIG. 2or 3, the equivalent electrostatic capacity and equivalent resistancevalue of components other than the display layers 8A, 8B, and 8C betweenthe writing electrodes 4 and 5 are Co and Ro, respectively, and theelectrostatic capacity and resistance value of the separating substrates6A and 6B are not included; in the display element of the example ofFIG. 4 or 7, the equivalent electrostatic capacity and equivalentresistance value of components other than the display layers 8A, 8B, and8C between the writing electrodes 24 and 25 are Co and Ro, respectively,and the electrostatic capacity and resistance value of the substrates 2and 3, and the light absorption layer 9 or light scattering layer 18 areincluded; in the display element of the example of FIG. 5 or 8, theequivalent electrostatic capacity and equivalent resistance value ofcomponents other than the display layers 8A, 8B, and 8C between thewriting electrodes 4 and 5 are Co and Ro, respectively, and theelectrostatic capacity and resistance value of the light absorptionlayer 9, the light scattering layer 18, and the photoconductive layer 15are included.

[0133] Therefore, in the display element 1 of the present invention, bycontrolling a distribution ratio of a voltage V applied from the writingapparatus 20 to the display layers 8A, 8B, and 8C, and electo-opticalresponses of the display layers 8A, 8B, and 8C to voltages Vxa, Vxb, andVxc actually applied to the display layers 8A, 8B, and 8C, theelecto-optical responses of the display layers 8A, 8B, and 8C to thevoltage V applied from the writing apparatus 20 can be set to a desiredlevel.

[0134] Specifically, the distribution ratio to the display layers 8A,8B, and 8C can be controlled by the dielectric constants of thecholesteric liquid crystals 11A, 11B, and 11C constituting the displaylayers 8A, 8B, and 8C. The electo-optical responses of the displaylayers 8A, 8B, and 8C can be controlled by the dielectric anisotropy,elasticity coefficient, and helical pitch of the cholesteric liquidcrystals 11A, 11B, and 11C constituting the display layers 8A, 8B, and8C, and further if a high polymer is added, the degree of an anchoringeffect in the interface of the high polymer and the liquid crystals,influenced by the structure of the high polymer and the phase separationprocess.

[0135] In the display element of the example of FIG. 5 or 8, when theamount of write light 28 is small, a resistance value of thephotoconductive layer 15 becomes large and a voltage (V−Vxo) actuallyapplied to the whole of the display layers 8A, 8B, and 8C decreases; andwhen the amount of write light 28 is large, a resistance value of thephotoconductive layer 15 becomes small and a voltage (V−Vxo) actuallyapplied to the whole of the display layers 8A, 8B, and 8C increases.Therefore, by controlling the light amount of write light 28, when avoltage V is applied across the writing electrodes 4 and 5 of thedisplay element 1 from the writing apparatus 20, a voltage (V−Vxo)actually applied to the whole of the display layers 8A, 8B, and 8C canbe controlled.

First Example of Electo-optical Response and a Writing Method

[0136] The electo-optical responses of the display layers 8A, 8B, and 8Cof the display element 1 of the present invention to a voltage V appliedfrom the writing apparatus 20 are set as shown in FIG. 10, as a firstexample.

[0137] That is, assuming that threshold voltages shown in FIG. 27 of thedisplay layer 8A are Vpf90(A), Vpf10(A), Vfh10(A), and Vfh90(A);threshold voltages shown in FIG. 27 of the display layer 8B areVpf90(B), Vpf10(B), Vfh10(B), and Vfh90(B); and threshold voltages shownin FIG. 27 of the display layer 8C are Vpf90(C), Vpf10(C), Vfh10(C), andVfh90(C), an expression (6) shown below is set.

Vpf90(C)<Vpf10(C)<Vpf90(B)<Vpf10(B)<Vfh10(C)<Vfh90(C)<Vpf90(A)<Vpf10(A)<Vfh10(B)<Vfh90(B)<Vfh10(A)<Vfh90(A)  (6)

[0138] The order in which the display layers are stacked is not limitedto the examples of FIGS. 1 to 8. That is, regardless of the order inwhich the display layers are stacked, when the three display layers aredefined as 8A, 8B, and 8C in descending order of threshold voltagesVpf90, Vpf10, Vfh10, and Vfh90, arrangements are made so that thefollowing expression is satisfied

Vfh90(C)<Vpf90(A)  (6a)

[0139] and there are no other threshold voltages between both.

[0140] In this case, as shown in FIG. 10, voltages Va, Vb, Vc, Vd, Ve,Vf, and Vg are defined as follows:

[0141] Va: Voltage below Vpf90(C)

[0142] Vb: Voltage between Vpf10(C) and Vpf90(B)

[0143] Vc: Voltage between Vpf10(B) and Vfh10(C)

[0144] Vd: Voltage between Vfh90(C) and Vpf90(A)

[0145] Ve: Voltage between Vpf10(A) and Vfh10(B)

[0146] Vf: Voltage between Vfh90(B) and Vfh10(A)

[0147] Vg: Voltage above Vfh90(A)

[0148] When there are a refresh period Tr, a select period Ts, and afollowing non-voltage display period Td as shown in FIG. 11, by thewriting apparatus 20, a writing signal representing a voltage selectedfrom the above-described seven voltage levels Va to Vg, based on inputimage data, is applied between the writing electrodes 4 and 5 or betweenthe electrodes 24 and 25, holding the relation that a voltage Vr in therefresh period Tr is greater than a voltage Vs in the select period Ts.

[0149]FIG. 12 shows, in this case, the orientation states of the displaylayers 8A, 8B, and 8C by combinations of refresh voltage Vr and selectvoltage Vs, wherein “p” designates a planer state; “f”, a focal conicstate; and “?”, an undecided state depending on a state before a writesignal is applied. The orientation states indicate the display layers8C, 8B, and 8A from the left in that order.

[0150] As is apparent from the above, according to the first example onthe display element of the present invention and electo-opticalresponses and a writing method, the following eight types of orientationstates are obtained.

[0151] (1) All of the display layers 8A, 8B, and 8C are the planerstate.

[0152] (2) All of the display layers 8A, 8B, and 8C are the focal conicstate.

[0153] (3) The display layer 8A is the planer state, and the displaylayers 8B and 8C are the focal conic state.

[0154] (4) The display layer 8B is the planer state, and the displaylayers 8A and 8C are the focal conic state.

[0155] (5) The display layer 8C is the planer state, and the displaylayers 8A and 8B are the focal conic state.

[0156] (6) The display layers 8A and 8B are the planer state, and thedisplay layer 8C is the focal conic state.

[0157] (7) The display layers 8A and 8C are the planer state, and thedisplay layer 8B is the focal conic state.

[0158] (8) The display layers 8B and 8C are the planer state, and thedisplay layer 8A is the focal conic state.

[0159] Therefore, in the display elements of the first embodiment in theexamples of FIGS. 1 to 5, for example, on the assumption that thedisplay layers 8A, 8B, and 8C selectively reflect blue light, greenlight, and red light, respectively, as shown in FIG. 13 (“T” in thedrawing indicates that a corresponding display layer is a transmissionstate due to the focal conic state), the display elements can assume thefollowing eight display states, so that eight colors—white, black, blue,green, red, cyan, magenta, and yellow—can be displayed within one pixel.

[0160]FIG. 13A White (W) by additive color mixture of blue, green, andred lights is displayed by a writing signal satisfying relations ofVr=Vg and Vs=Va.

[0161]FIG. 13B Black (Bk) is displayed by, e.g., a writing signalsatisfying relations of Vr=Ve and Vs=Vc.

[0162]FIG. 13C Blue (B) is displayed by a writing signal satisfyingrelations of Vr=Vg and Vs=Vc.

[0163]FIG. 13D Green (G) is displayed by a writing signal satisfyingrelations of Vr=Vf and Vs=Vb.

[0164]FIG. 13E Red (R) is displayed by, e.g., a writing signalsatisfying relations of Vr=Ve and Vs=Va.

[0165]FIG. 13F Cyan (C) by additive color mixture of blue and greenlights is displayed by a writing signal satisfying relations of Vr=Vgand Vs=Vb.

[0166]FIG. 13G Magenta (M) by additive color mixture of blue and redlights is displayed by a writing signal satisfying relations of Vr=Vgand Vs=Vd.

[0167]FIG. 13H Yellow (Y) by additive color mixture of green and redlights is displayed by, e.g., a writing signal satisfying relations ofVr=Vf and Vs=Va.

[0168] Moreover, a full-color display can be made by performing areagradation such as the dither method and the error diffusion method.

[0169] The relationship between the magnitudes of threshold voltages ofthe display layers 8A, 8B, and 8C and selective reflection colors can bearbitrarily set without being limited to the above examples.

[0170] On the other hand, in the display elements of the secondembodiment in the examples of FIGS. 6 to 8, as the dichroic dyes 17A,17B, and 17C, by using positive dichroic dyes that a light absorptionindex in a molecular long axis direction is larger than that in amolecular short axis direction, in the planer state, a display layerabsorbs incident light, and in the focal conic state, the display layerallows almost all of incident light to transmit therethrough.

[0171] Therefore, in the display elements of the second embodiment, forexample, if the display layer 8A is added with a cyan dichroic dyeselectively absorbing red light as a dichroic dye 17A, the display layer8B is added with a magenta dichroic dye selectively absorbing greenlight as a dichroic dye 17B, and the display layer 8C is added with ayellow dichroic dye selectively absorbing blue light as a dichroic dye17C, as shown in FIG. 14 (“T” in the drawing indicates that acorresponding display layer is a transmission state due to the focalconic state), the display elements can assume the following eightdisplay states, so that eight colors—white, black, blue, green, red,cyan, magenta, and yellow—can be displayed within one pixel.

[0172]FIG. 14A Black (Bk) by subtractive color mixture of cyan, magenta,and yellow lights is displayed by a writing signal satisfying relationsof Vr=Vg and Vs=Va.

[0173]FIG. 14B White (W) is displayed by, e.g., a writing signalsatisfying relations of Vr=Ve and Vs=Vc.

[0174]FIG. 14C Cyan (C) is displayed by a writing signal satisfyingrelations of Vr=Vg and Vs=Vc.

[0175]FIG. 14D Magenta (M) is displayed by a writing signal satisfyingrelations of Vr=Vf and Vs=Vb.

[0176]FIG. 14E Yellow (Y) is displayed by, e.g., a writing signalsatisfying relations of Vr=Ve and Vs=Va.

[0177]FIG. 14F Blue (B) by subtractive color mixture of cyan and magentalights is displayed by a writing signal satisfying relations of Vr=Vgand Vs=Vb.

[0178]FIG. 14G Green (G) by subtractive color mixture of cyan and yellowlights is displayed by a writing signal satisfying relations of Vr=Vgand Vs=Vd.

[0179]FIG. 14H Red (R) by additive color mixture of green and red lightsis displayed by, e.g., a writing signal satisfying relations of Vr=Vfand Vs=Va.

[0180] Moreover, a full-color display can be made by performing areagradation such as the dither method and the error diffusion method.

[0181] The relationship between the magnitudes of threshold voltages ofthe display layers 8A, 8B, and 8C and selective absorption colors can bearbitrarily set without being limited to the above examples.

Second Example of Electo-optical Response and a Writing Method

[0182] The electo-optical responses of the display layers 8A, 8B, and 8Cof the display element 1 of the present invention to a voltage V appliedfrom the writing apparatus 20 are set by an expression (7) below asshown in FIG. 15, as a second example.

Vpf90(C)<Vpf10(C)<Vfh10(C)<Vfh90(C)<Vpf90(B)<Vpf10(B)<Vpf90(A)<Vpf10(A)<Vfh10(B)<Vfh90(B)<Vfh10(A)<Vfh90(A)  (7)

[0183] That is, regardless of the order in which the display layers arestacked, when the three display layers are defined as 8A, 8B, and 8C indescending order of threshold voltages Vpf90, Vpf10, Vfh10, and Vfh90,arrangements are made so that the following expression is satisfied.

Vfh90(C)<Vpf90(B)<Vpf10(B)<Vpf90(A)  (7a)

[0184] That is, in the relation of the expression (6a), Vpf90(B) andVpf10(B) are put between Vfh90(C) and Vpf90(A).

[0185] In this case, as shown in FIG. 15, voltages Va, Vb, Vc, Vd, Ve,Vf, and Vg are defined as follows:

[0186] Va: Voltage below Vpf90(C)

[0187] Vb: Voltage between Vpf10(C) and Vfh10C)

[0188] Vc: Voltage between Vfh90(C) and Vpf90(B)

[0189] Vd: Voltage between Vpf10(B) and Vpf90(A)

[0190] Ve: Voltage between Vpf10(A) and Vfh10(B)

[0191] Vf: Voltage between Vfh90(B) and Vfh10(A)

[0192] Vg: Voltage above Vfh90(A)

[0193] When there are a preset period Tp, a refresh period Tr, a selectperiod Ts, and a following non-voltage display period Td as shown inFIG. 16, wherein a voltage Vp in the preset period is equal to the aboveVg, by the writing apparatus 20, a writing signal representing a voltageselected from the above-described seven voltage levels Va to Vg, basedon input image data, is applied between the writing electrodes 4 and 5or between the electrodes 24 and 25, holding the relation that a voltageVr in the refresh period Tr is greater than a voltage Vs in the selectperiod Ts.

[0194]FIG. 17 shows, in this case, the orientation states of the displaylayers 8A, 8B, and 8C by combinations of refresh voltage Vr and selectvoltage Vs, wherein “p” designates a planer state and “f” designates afocal conic state. The orientation states indicate the display layers8C, 8B, and 8A from the left in that order.

[0195] As is apparent from the above, according to the second example onthe display element of the present invention and electo-opticalresponses and a writing method, eight types of orientation states areobtained, like the first example.

[0196] Therefore, in the display elements of the first embodiment in theexamples of FIGS. 1 to 5, for example, on the assumption that thedisplay layers 8A, 8B, and 8C selectively reflect blue light, greenlight, and red light, respectively, as shown in FIG. 13, the displayelements can assume the following eight display states, so that eightcolors—white, black, blue, green, red, cyan, magenta, and yellow—can bedisplayed within one pixel.

[0197]FIG. 13A White (W) by additive color mixture of blue, green, andred lights is displayed by, e.g., a writing signal satisfying relationsof Vp=Vg, Vr=Vg, and Vs=Va.

[0198]FIG. 13B Black (Bk) is displayed by a writing signal satisfyingrelations of Vp=Vg, Vr=Ve, and Vs=Vb.

[0199]FIG. 13C Blue (B) is displayed by a writing signal satisfyingrelations of Vp=Vg, Vr=Vd, and Vs=Vb.

[0200]FIG. 13D Green (G) is displayed by a writing signal satisfyingrelations of Vp=Vg, Vr=Vf, and Vs=Vb.

[0201]FIG. 13E Red (R) is displayed by, e.g., a writing signalsatisfying relations of Vp=Vg, Vr=Ve, and Vs=Va.

[0202]FIG. 13F Cyan (C) by additive color mixture of blue and greenlights is displayed by a writing signal satisfying relations of Vp=Vg,Vr=Vg, and Vs=Vb.

[0203]FIG. 13G Magenta (M) by additive color mixture of blue and redlights is displayed by, e.g., a writing signal satisfying relations ofVp=Vg, Vr=Vg, and Vs=Vd.

[0204]FIG. 13H Yellow (Y) by additive color mixture of green and redlights is displayed by, e.g., a writing signal satisfying relations ofVp=Vg, Vr=Vf, and Vs=Va.

[0205] In the display elements of the second embodiment in the examplesof FIGS. 6 to 8, for example, if the display layer 8A is added with acyan dichroic dye selectively absorbing red light as a dichroic dye 17A,the display layer 8B is added with a magenta dichroic dye selectivelyabsorbing green light as a dichroic dye 17B, and the display layer 8C isadded with a yellow dichroic dye selectively absorbing blue light as adichroic dye 17C, as shown in FIG. 14, the display elements can assumethe following eight display states, so that eight colors—white, black,blue, green, red, cyan, magenta, and yellow—can be displayed within onepixel.

[0206]FIG. 14A Black (Bk) by subtractive color mixture of cyan, magenta,and yellow lights is displayed by a writing signal satisfying relationsof Vp=Vg, Vr=Vg, and Vs=Va.

[0207]FIG. 14B White (W) is displayed by, e.g., a writing signalsatisfying relations of Vp=Vg, Vr=Ve, and Vs=Vb.

[0208]FIG. 14C Cyan (C) is displayed by a writing signal satisfyingrelations of Vp=Vg, Vr=Vd, and Vs=Vb.

[0209]FIG. 14D Magenta (M) is displayed by a writing signal satisfyingrelations of Vp=Vg, Vr=Vf, and Vs=Vb.

[0210]FIG. 14E Yellow (Y) is displayed by, e.g., a writing signalsatisfying relations of Vp=Vg, Vr=Ve, and Vs=Va.

[0211]FIG. 14F Blue (B) by subtractive color mixture of cyan and magentalights is displayed by, e.g., a writing signal satisfying relations ofVp=Vg, Vr=Vg, and Vs=Vb.

[0212]FIG. 14G Green (G) by subtractive color mixture of cyan and yellowlights is displayed by, e.g., a writing signal satisfying relations ofVp=Vg, Vr=Vg, and Vs=Vd.

[0213]FIG. 14H Red (R) by additive color mixture of magenta and yellowlights is displayed by, e.g., a writing signal satisfying relations ofVp=Vg, Vr=Vf, and Vs=Va.

[0214] The present example is the same as the first example, in that afull-color display can be made by performing area gradation such as thedither method and the error diffusion method, and the relationshipbetween the magnitudes of threshold voltages of the display layers 8A,8B, and 8C and the selective reflection colors of the first embodimentor the selective absorption colors of the second embodiment can bearbitrarily set without being limited to the above examples.

Third Example of Electo-optical Response and a Writing Method

[0215] The electo-optical responses of the display layers 8A, 8B, and 8Cof the display element 1 of the present invention to a voltage V appliedfrom the writing apparatus 20 are set by an expression (8) below asshown in FIG. 18, as a third example.

Vpf90(C)<Vpf10(C)<Vfh10(C)<Vfh90(C)<Vpf90(B)<Vpf10(B)<Vfh10(B)<Vfh90(B)<Vpf90(A)<Vpf10(A)<Vfh10(A)<Vfh90(A)  (8)

[0216] That is, regardless of the order in which the display layers arestacked, when the three display layers are defined as 8A, 8B, and 8C indescending order of threshold voltages Vpf90, Vpf10, Vfh10, and Vfh90,arrangements are made so that the following expression is satisfied.

Vfh90(C)<Vpf90(B)<Vpf10(B)<Vfh10(B)<Vfh90(B)<Vpf90(A)  (8a)

[0217] That is, in the relation of the expression (6a), Vpf90(B),Vpf10(B), Vfh10(B), and Vfh90(B) are put between Vfh90(C) and Vpf90(A).

[0218] In this case, as shown in FIG. 18, voltages Va, Vb, Vc, Vd, Ve,Vf, and Vg are defined as follows:

[0219] Va: Voltage below Vpf90(C)

[0220] Vb: Voltage between Vpf10(C) and Vfh10(C)

[0221] Vc: Voltage between Vfh90(C) and Vpf90(B)

[0222] Vd: Voltage between Vpf10(B) and Vfh10(B)

[0223] Ve: Voltage between Vfh90(B) and Vpf90(A)

[0224] Vf: Voltage between Vpf10(A) and Vfh10(A)

[0225] Vg: Voltage above Vfh90(A)

[0226] When there are a first preset period Tp1, a second preset periodTp2, a refresh period Tr, a select period Ts, and a followingnon-voltage display period Td as shown in FIG. 19, wherein a voltage Vp1in the first preset period Tp1 is equal to the above Vg and a voltageVp2 in the second preset period Tp2 is equal to the above Vf or Ve, bythe writing apparatus 20, a writing signal representing a voltageselected from the above-described seven voltage levels Va to Vg, basedon input image data, is applied between the writing electrodes 4 and 5or between the electrodes 24 and 25, holding the relation that a voltageVr in the refresh period Tr is greater than a voltage Vs in the selectperiod Ts.

[0227]FIG. 20 and FIG. 21 show the orientation states of the displaylayers 8A, 8B, and 8C by combinations of refresh voltage Vr and selectvoltage Vs, in which FIG. 20 assumes Vp2=Vf, and FIG. 21 assumesVpf2=Ve. “p” designates a planer state and “f” designates a focal conicstate. The orientation states indicate the display layers 8C, 8B, and 8Afrom the left in that order.

[0228] As is apparent from the above, according to the third example onthe display element of the present invention and electo-opticalresponses and a writing method, eight types of orientation states areobtained, like the first and second examples.

[0229] Therefore, in the display elements of the first embodiment in theexamples of FIGS. 1 to 5, for example, on the assumption that thedisplay layers 8A, 8B, and 8C selectively reflect blue light, greenlight, and red light, respectively, as shown in FIG. 13, the displayelements can assume the following eight display states, so that eightcolors—white, black, blue, green, red, cyan, magenta, and yellow—can bedisplayed within one pixel.

[0230]FIG. 13A White (W) by additive color mixture of blue, green, andred lights is displayed by, e.g., a writing signal satisfying relationsof Vp1=Vg, Vp2=Vf, Vr=Vg, and Vs=Va.

[0231]FIG. 13B Black (Bk) is displayed by a writing signal satisfyingrelations of Vp1=Vg, Vp2=Vf, Vr=Vd, and Vs=Vb.

[0232]FIG. 13C Blue (B) is displayed by a writing signal satisfyingrelations of Vp1=Vg, Vp2=Ve, Vr=Vd, and Vs=Vb.

[0233]FIG. 13D Green (G) is displayed by, e.g., a writing signalsatisfying relations of Vp=Vg, Vp2=Vf, Vr=Vf, and Vs=Vb.

[0234]FIG. 13E Red (R) is displayed by, e.g., a writing signalsatisfying relations of Vp1=Vg, Vp2=Vf, Vr=Vd, and Vs=Va.

[0235]FIG. 13F Cyan (C) by additive color mixture of blue and greenlights is displayed by, e.g., a writing signal satisfying relations ofVp1=Vg, Vp2=Vf, Vr=Vg, and Vs=Vb.

[0236]FIG. 13G Magenta (M) by additive color mixture of blue and redlights is displayed by, e.g., a writing signal satisfying relations ofVp1=Vg, Vp2=Vf, Vr=Vg, and Vs=Vd.

[0237]FIG. 13H Yellow (Y) by additive color mixture of green and redlights is displayed by, e.g., a writing signal satisfying relations ofVp1=Vg, Vp2=Vf, Vr=Vf, and Vs=Va.

[0238] In the display elements of the second embodiment in the examplesof FIGS. 6 to 8, for example, if the display layer 8A is added with acyan dichroic dye selectively absorbing red light as a dichroic dye 17A,the display layer 8B is added with a magenta dichroic dye selectivelyabsorbing green light as a dichroic dye 17B, and the display layer 8C isadded with a yellow dichroic dye selectively absorbing blue light as adichroic dye 17C, as shown in FIG. 14, the display elements can assumethe following eight display states, so that eight colors—white, black,blue, green, red, cyan, magenta, and yellow—can be displayed within onepixel.

[0239]FIG. 14A Black (Bk) by subtractive color mixture of cyan, magenta,and yellow lights is displayed by a writing signal satisfying relationsof Vp1=Vg, Vp2=Vf, Vr=Vg, and Vs=Va.

[0240]FIG. 13B White (W) is displayed by, e.g., a writing signalsatisfying relations of Vp1=Vg, Vp2=Vf, Vr=Vd, and Vs=Vb.

[0241]FIG. 13C Cyan (C) is displayed by a writing signal satisfyingrelations of Vp1=Vg, Vp2=Ve, Vr=Vd, and Vs=Vb.

[0242]FIG. 13D Magenta (M) is displayed by, e.g., a writing signalsatisfying relations of Vp1=Vg, Vp2=Vf, Vr=Vf, and Vs=Vb.

[0243]FIG. 13E Yellow (Y) is displayed by, e.g., a writing signalsatisfying relations of Vp1=Vg, Vp2=Vf, Vr=Vd, and Vs=Va.

[0244]FIG. 13F Blue (B) by subtractive color mixture of cyan and magentalights is displayed by, e.g., a writing signal satisfying relations ofVp1=Vg, Vp2=Vf, Vr=Vg, and Vs=Vb.

[0245]FIG. 13G Green (G) by subtractive color mixture of cyan and yellowlights is displayed by, e.g., a writing signal satisfying relations ofVp1=Vg, Vp2=Vf, Vr=Vg, and Vs=Vd.

[0246]FIG. 13H Red (R) by additive color mixture of magenta and yellowlights is displayed by, e.g., a writing signal satisfying relations ofVp1=Vg, Vp2=Vf, Vr=Vf, and Vs=Va.

[0247] The present example is the same as the first and second examples,in that a full-color display can be made by performing area gradationsuch as the dither method and the error diffusion method, and therelationship between the magnitudes of threshold voltages of the displaylayers 8A, 8B, and 8C and the selective reflection colors of the firstembodiment or the selective absorption colors of the second embodimentcan be arbitrarily set without being limited to the above examples.

Other Embodiment or Example

[0248] In the examples of FIGS. 1 to 8, only three display layers 8A,8B, and 8C are stacked between the substrates 2 and 3. However, fourdisplay layers or more may be stacked.

[0249] For example, as in the first embodiment, when a display is madeby selective reflection of cholesteric liquid crystals, each of displaylayers selectively reflecting blue, green, and red lights, respectivelymay be constituted by a display layer including a cholesteric liquidcrystal having a clockwise helical torsion direction and a display layerincluding a cholesteric liquid crystal having a counterclockwise helicaltorsion direction. In this case, although a total of six display layersare stacked between a pair of substrates, two display layers selectivelyreflecting the same color light are adapted to exhibit the sameelecto-optical response. Thereby, a display having a higher reflectivitycan be made.

[0250] In addition to display layers selectively reflecting blue, green,and red light, respectively, provided between a pair of substrates, adisplay layer selectively reflecting yellow light may also be stacked.

[0251] As in the second embodiment in the examples of FIGS. 6 to 8, whena display is made by selective absorption, instead of adding dichroicdyes 17A, 17 b, and 17C to the cholesteric liquid crystals 11A, 11B, and11C as described above, as the cholesteric liquid crystals 11A, 11B, and11C, those having dichroism that selectively absorb mutually differentcolor lights can be used.

[0252] In the above-described examples on the electo-optical responsesof cholesteric liquid crystals constituting display layers, the state inwhich a normalized reflectivity is 90 or more is defined as a selectivereflection state, and the state in which a normalized reflectivity is 10or more is defined as a transmission state. However, in principle, thestate in which a normalized reflectivity is equal to or greater than avalue exceeding 50 may be defined as a selective reflection state, andthe state in which a normalized reflectivity is less than 50 may bedefined as a transmission state. However, definitions to narrow theselective reflection state and the transmission state are more desirablein terms of display characteristics.

Experiment Examples Experiment Example 1

[0253] Since it is impossible to evaluate the characteristics of each ofstacked display layers, in an experiment example 1, a display cellhaving a blue display layer, a display cell having a green displaylayer, and a display cell having a red display layer were fabricated,and writing was performed in the state in which the display cells wereconnected in series, to measure display characteristics, the state beingelectrically equivalent to the case where the display layers werestacked.

[0254] As a cholesteric liquid crystal to constitute the blue displaylayer, a nematic liquid crystal (MLC2037 made by Merck Ltd.), a chiralagent 1 (CB15 made by Merck Ltd.), and a chiral agent 2 (R1011 made byMerck Ltd.) were mixed at the ratio of 73.0 wt %, 22.5 wt %, and 4.5 wt%, respectively.

[0255] As a cholesteric liquid crystal to constitute the green displaylayer, a nematic liquid crystal (MLC2038 made by Merck Ltd.), the chiralagent 1 (CB15 made by Merck Ltd.), and the chiral agent 2 (R1011 made byMerck Ltd.) were mixed at the ratio of 78.0 wt %, 18.3 wt %, and 3.7 wt%, respectively.

[0256] As a cholesteric liquid crystal to constitute the red displaylayer, a nematic liquid crystal (ZLI3806 made by Merck Ltd.), the chiralagent 1 (CB15 made by Merck Ltd.), and the chiral agent 2 (R1011 made byMerck Ltd.) were mixed at the ratio of 78.4 wt %, 18.0 wt %, and 3.6 wt%, respectively.

[0257] A material with a 15.0 wt % polymer precursor (NOA65 made byNorland, Inc.) added to a blue cholesteric liquid crystal was injectedinto an empty cell (made by EHC) by capillarity wherein the empty cellopposes at a gap of 5 μm a pair of glass substrates each having an ITOtransparent electrode 1.1 mm thick. UV light of 50 mW/cm² (365 nm) wasapplied for 30 seconds so that a light absorption layer of black resinwas formed on the back of one glass substrate to obtain a blue displaycell.

[0258] In the same manner, a green display cell and a red display cellwere created.

[0259] The obtained blue, green, and red display cells were connected inseries, and using a writing apparatus including an arbitrary waveformgenerator (made by Biomation, Inc.) and a high-voltage power supply(made by TREK INC.), a 1-kHz refresh signal was applied for 200 ms and a1-kHz select signal was applied for 200 ms. The display state of eachdisplay cell was measured using an integrating sphere typespectrophotometer (made by Minolta Co., Ltd.).

[0260]FIG. 22 shows a change in normalized reflectivity of each displaylayer when any select voltage is applied after a 700-V refresh voltageis applied.

[0261] It was confirmed that a writing method of the present inventioncould be achieved by controlling a writing signal so that, e.g., writingvoltages shown in FIG. 23 were applied to the whole of the displaylayers, based on the measurement results of FIG. 22.

Experiment Example 2

[0262] In an experiment example 2, a three-layer display element wasfabricated using the same cholesteric liquid crystals as in theexperiment example 1 to measure display characteristics.

[0263] A spherical spacer (Micropearl SP-205 made by Sekisui FineChemical, Inc.) 5 μm in diameter was wet-dispersed on a glass substrate1.1 mm thick (7059 made by Corning) provided with an ITO transparentelectrode, a material with a 15.0 wt % polymer precursor (NOA65 made byNorland, Inc.) added to a blue cholesteric liquid crystal was dropped, aspherical spacer (Micropearl SP-205 made by Sekisui Fine Chemical, Inc.)5 μm in diameter was wet-dispersed on a single side, and a PET film 4.5μm thick (LUMIRROR made by Toray Industries, Inc.) supported by aplastic frame was tightly bonded so that it contacts with anon-dispersion surface of the spacer.

[0264] Moreover, on top of the PET film, a material with a 15.0 wt %polymer precursor (NOA65 made by Norland, Inc.) added to a bluecholesteric liquid crystal was dropped, a spherical spacer (MicropearlSP-205 made by Sekisui Fine Chemical, Inc.) 5 μm in diameter waswet-dispersed on a single side, and a PET film 4.5 μm thick (LUMIRRORmade by Toray Industries, Inc.) supported by a plastic frame was tightlybonded so that it contacts with a non-dispersion side of the spacer.

[0265] Moreover, on top of the PET film, a material with a 15.0 wt %polymer precursor (NOA65 made by Norland, Inc.) added to a redcholesteric liquid crystal was dropped, and a glass substrate 1.1 mmthick (7059 made by Corning) provided with an ITO transparent electrodewas tightly bonded so that the electrode side would contact the liquidcrystal.

[0266] UV light of 50 mW/cm was applied for 30 seconds, and a lightabsorption layer of black resin was formed on the back of a glasssubstrate of the red display layer, so that a display element in whichdisplay layers of three colors were stacked was obtained.

[0267] Using a writing apparatus including an arbitrary waveformgenerator (made by Biomation, Inc.) and a high-voltage power supply(made by TREK, INC.), the obtained display element was applied with a1-kHz refresh signal for 200 ms and with a 1-kHz select signal for 200ms. The display state of the display element was measured using anintegrating sphere type spectrophotometer (made by Minolta Co., Ltd.).

[0268]FIG. 24 shows reflection spectrum distributions in the case wherea writing signal is controlled so that the writing voltages shown inFIG. 23 are applied to the whole of the display layers in considerationof voltage drop by a separating substrate; and FIG. 25 shows colorreproducing areas in the a*b* display color system in that case. It wasconfirmed that the display element of the experiment example 2 coulddisplay eight colors—white, black, blue, green, red, cyan, magenta, andyellow—within one pixel.

Experiment Example 3

[0269] In an experiment example 3, a display element of PDLC structurewas fabricated using the same cholesteric liquid crystals as in thefirst experiment example 1.

[0270] A 10 wt % aqueous solution of PVA 1000 (made by Wako PureChemical Industries Ltd.) was mixed with cholesteric liquid crystalsrespectively at the ratio of 1 to 2.5, and the mixtures were stirred at15,000 rpm for three minutes using a homogenizer (made by OMNI) toproduce blue, green, and red emulsions.

[0271] A coating of the viscosity-adjusted blue emulsion was applied ona PET film 125 μm thick provided with an ITO transparentelectrode(Hybeam made by Toray Industries, Inc.), using a doctor blade(Gardner, Inc.) and was dried to form a blue display layer about 10 μmthick of PDLC structure.

[0272] To stack display layers of three colors on top of each other,green and red display layers were successively formed on the bluedisplay layer by the same method, and a PET film 125 μm thick providedwith an ITO transparent electrode(Hybeam made by Toray Industries, Inc.)was tightly bonded onto the red display layer, using a laminator.

[0273] A light absorption layer of black resin was formed on the back ofthe PET film of a non-display side, and a display element in whichthree-color display layers of PDLC structure were stacked was obtained.

Experiment Example 4

[0274] In an experiment example 4, a display element of micro capsulestructure was fabricated using the same cholesteric liquid crystals asin the experiment example 1.

[0275] Each cholesteric liquid crystal was dispersed in a 0.25 wt %aqueous sodium dodecyl benzene sulfate, using an SPG membraneemulsification apparatus having a pore diameter of 3.3 μm (made by IseChemicals Corp.), and blue, green, and red emulsions were produced.

[0276] The obtained emulsions were mixed with a melamine-formaldehydeprepolymer (MX-035 made by SANWA CHEMICAL CO., LTD.) at the ratio of 2to 1 with respect to cholesteric liquid crystals, respectively, and a 10wt % aqueous solution of acetic acid was dropped to adjust pH to 4.5.

[0277] The obtained mixtures were stirred at 70° C. for 60 minutes and amelamine-formaldehyde prepolymer was polymerized in situ to obtain blue,green, and red micro capsules about 10 μm in diameter.

[0278] The blue micro capsule was dispersed in a binder of a PVA aqueoussolution, a coating was produced on a PET film 125 μm thick (LUMIRRORmade by Toray Industries, Inc.) by using a doctor blade (made byGardner, Inc.), and after it was dried, a blue display layer about 15 μmthick was formed.

[0279] To stack display layers of three colors on top of each other,green and red display layers were successively formed on the bluedisplay layer by the same method, and a PET film 125 μm thick (LUMIRRORmade by Toray Industries, Inc.) was tightly bonded onto the red displaylayer, using a laminator.

[0280] A light absorption layer of black resin was formed on the back ofthe PET film of a non-display side, and a display element in whichthree-color display layers of micro capsule structure were stacked wasobtained.

Experiment Example 5

[0281] In an experiment example 5, a display element having aphotoconductive layer on the substrate of the non-display side of theexperiment example 2 was fabricated.

[0282] On a glass substrate 1.1 mm thick (7059 made by Corning) providedwith an ITO transparent electrode, as a charge generating layer, asolution of 2.1 wt % chlorogallium phthalocyanine, 1.8 wt % vinylchloride/vinyl acetate copolymer resin, 31.7 wt % n-butyl acetate, 64.4wt % xylene was applied to produce a coating 0.25 μm thick by the dipcoating method.

[0283] Moreover, on top of the charge generation layer, as a chargetransport layer, a solution of a 7.2 wt %3,3′-dimethyl-N,N,N′,N′-tetrakis(4-methylphenyl)-1,1′-biphenyl-4,4′-diamine, 10.8 wt % poly(4,4′-cyclohexylden diphenylene carbonate, and 82 wt % monochlorobenzenewas applied to produce a coating 3 μm thick by the dip coating method.

[0284] Moreover, a light absorption layer of black resin was formed onthe photoconductive layer having the charge generation layer and thecharge transport layer.

[0285] With the obtained substrate as the non-display side, a displayelement in which display layers of three colors are stacked on the lightabsorption layer was obtained by the same method as in the experimentexample 2.

Experiment Example 6

[0286] In the experiment example 6, a display element in which dichroicdyes are added to the display layers of the experiment example 2 wasfabricated.

[0287] As a material to constitute a cyan display layer, a 0.5 wt %dichroic dye (SI497 made by Mitsui Toatsu Chemicals, Inc.) to absorbgreen light was added to the blue cholesteric liquid crystal used in theexperiment example 2.

[0288] As a material to constitute a magenta display layer, a 0.5 wt %dichroic dye (S618, made by Mitsui Toatsu Chemicals, Inc.) to absorb redlight was added to the red cholesteric liquid crystal used in theexperiment example 2.

[0289] As a material to constitute a yellow display layer, a 0.5 wt %dichroic dye (SI486 made by Mitsui Toatsu Chemicals, Inc.) to absorbblue light was added to the green cholesteric liquid crystal used in theexperiment example 2.

[0290] A display element in which display layers of three colors made ofcholesteric liquid crystals with dichroic dyes were stacked was obtainedwith the construction that the display layers of three colors arestacked using the obtained mixed materials of cyan, magenta, and yellowby the same method as in the experiment example 2, and a lightscattering layer of white resin is formed on the back of the substrateof the non-display side.

[0291] As described above, according to the present invention, in adisplay element in which plural display layers of three or more layersto display mutually different color lights are stacked within one pixeland which controls display states of the plural display layers byapplying a voltage from the outside of the plural display layers, eightcolors—white, black, blue, green, red, cyan, magenta, and yellow—can bedisplayed within one pixel, and a color reproduction area can beenlarged.

[0292] The entire disclosure of Japanese Patent Application No.2000-010148 filed on Jan. 14, 2000 including specification, claims,drawings and abstract is incorporated herein by reference in itsentirety.

What is claimed is:
 1. A display element comprising: three or moredisplay layers each including cholesteric liquid crystal, the displaylayers selectively reflecting lights of different peak wavelengths,respectively, being stacked within one pixel, and having a thresholdvoltage of orientation change of the cholesteric liquid crystalsdiffering from each other for voltage applied from the outside of thedisplay layers, wherein, among the three or more display layers, athreshold voltage of change from a planer state to a focal conic stateof the display layer having the highest threshold voltage is higher thana threshold voltage of change from a focal conic state to a homeotropicstate of the display layer having the lowest threshold voltage.
 2. Amethod of writing an image to the display element of claim 1 ,comprising the steps of: causing a writing signal to include at least arefresh period, a select period, and a following non-voltage displayperiod; and applying the writing signal to the display element of claim1 , wherein a voltage Vr in the refresh period is greater than a voltageVs in the select period.
 3. An apparatus that writes an image to thedisplay element of claim 1 , comprising: a signal applying unit applyinga writing signal which includes at least a refresh period, a selectperiod, and a following non-voltage display period, wherein a voltage Vrin the refresh period is greater than a voltage Vs in the select period.4. A display apparatus comprising: the display element of claim 1 ; anda writing apparatus that writes an image thereto, wherein the writingapparatus applies a writing signal which includes at least a refreshperiod, a select period, and a following non-voltage display period, anda voltage Vr in the refresh period is greater than a voltage Vs in theselect period.
 5. A display element comprising: three or more displaylayers including cholesteric liquid crystals selectively absorbinglights of different peak wavelengths, respectively, by adding dichroicdyes to the cholesteric liquid crystals or by the dichroism of thecholesteric liquid crystals themselves, the display layers being stackedwithin one pixel, and having threshold voltages of orientation change ofthe cholesteric liquid crystals differing from each other for a voltageapplied from the outside of the display layers, wherein, among the threeor more display layers, a threshold voltage of change from a planerstate to a focal conic state of the display layer having the highestthreshold voltage is higher than a threshold voltage of change from afocal conic state to a homeotropic state of the display layer having thelowest threshold voltage.
 6. A method of writing an image to the displayelement of claim 5 , comprising the steps of: causing a writing signalto include at least a refresh period, a select period, and a followingnon-voltage display period; and applying the writing signal to thedisplay element of claim 1 , wherein a voltage Vr in the refresh periodis greater than a voltage Vs in the select period.
 7. An apparatus thatwrites an image to the display element of claim 5 , comprising: a signalapplying unit applying a writing signal which includes at least arefresh period, a select period, and a following non-voltage displayperiod, wherein a voltage Vr in the refresh period is greater than avoltage Vs in the select period.
 8. A display apparatus comprising: thedisplay element of claim 1 ; and a writing apparatus that writes animage thereto, wherein the writing apparatus applies a writing signalwhich includes at least a refresh period, a select period, and afollowing non-voltage display period, and a voltage Vr in the refreshperiod is greater than a voltage Vs in the select period.